The TARDIS lands on board a spaceship from 28th-century Earth. Its three occupants at first appear to be dead but then suddenly revive. Captain Maitland (Lorne Cossette) explains that he and his crew members, Carol Richmond (Ilona Rodgers) and John (Stephen Dartnell), have come under the influence of a race of telepathic aliens called Sensorites, who plunge them into catatonic trances for indeterminate periods.

Through telepathic communication with Susan, two Sensorites (Ken Tyllsen, Joe Greig) convey a request from their leader for a meeting with the travellers. While Maitland and Barbara remain on the spaceship, the Doctor, Ian, Susan, Carol and John all journey down to the creatures' planet, the Sense-Sphere, for talks.

The Sensorites' First Elder (Eric Francis) and Second Elder (Bartlett Mullins) explain that they know the humans have discovered the metal molybdenum on the Sense-Sphere and that they are afraid of being exploited. They fear humans as, since an earlier expedition left, many of their race have died from a mysterious disease. Despite opposition from the devious City Administrator (Peter Glaze), the Doctor is given permission to try to learn the cause of the deaths.

After carrying out extensive tests, the Doctor discovers that the deaths are due not to a disease at all, but to deadly nightshade poison in one of the city's aqueducts. He tracks down the culprits to a cave system near the aqueduct. They are three spacemen (John Bailey, Martyn Huntley, Giles Phibbs) left behind from the earlier expedition, who have become deranged after experimenting with telepathy. The City Administrator is arrested and banished after his attempts to cause mistrust between the Sensorites and humans are brought to light.

Synopsis from Doctor Who: The First Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos